Permselective membranes for gas separation are known and used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, the upgrading of natural gas streams to pipeline quality specifications (e.g., removal of carbon dioxide, hydrogen sulfide, and nitrogen from raw natural gas), and the recovery of hydrogen from various petrochemical and oil refining streams (e.g., separation of hydrogen from methane, ethane, ethylene, or carbon monoxide). Preferred membranes for industrial gas separations exhibit a combination of high flux and high permselectivity.
The permeability of a gas A, PA through a membrane is often expressed as
 PA=DA×SA
where DA is the effective diffusivity of the gas through the membrane material, and SA is the solubility coefficient of the gas in the material. The ideal selectivity of a membrane for component A relative to component B, αA/B, is the ratio of permeabilities of the two components:       α          A      /      B        =                    [                  P          A                ]                    [                  P          B                ]              =                            [                      D            A                    ]                          [                      D            B                    ]                    ×                        [                      S            A                    ]                          [                      S            B                    ]                    
where, DA/DB is the diffusivity selectivity, which is the ratio of diffusion coefficients of components A and B. The ratio of solubility coefficients of components A and B, SA/SB, is the solubility selectivity. Solubility selectivity is controlled by the relative affinity of the gas molecules to the polymer of the membrane; whereas, diffusivity selectivity is governed primarily by the relative sizes of the gas molecules and the sieving ability of the polymer matrix.
Current membranes used for industrial gas separation and/or concentration is primarily based on stiff-chain, rigid, glassy materials. The diffusivity component of the gas tends to be the controlling factor, and the ability of gas molecules to permeate is very size dependent. In such membrane materials, smaller gas molecules such as helium and hydrogen are more permeable than larger molecules such as oxygen, nitrogen, and methane. For rubbery or elastomeric polymers, the polymer chains are more flexible and less discriminating by molecular size. Solubility effects generally dominate selectivity in these cases. Permeability for rubbery polymers is generally much greater than for glassy, more rigid polymers. Generally, an inverse relationship between gas permeation rate and selectivity has been observed with most polymeric membranes. This relationship is generally observed for all glassy high glass-transition temperature polymers and for rubbery polymers. Consequently, prior-art gas separation membranes tend to exhibit either high gas permeation rates at the sacrifice of high permselectivity or the inverse. It would be highly desirable for gas separation membranes to exhibit both high gas permeation rates and high permselectivity. Further, it is desirable for such materials to be easily fabricated into appropriate membrane structures. An application where membranes have been used commercially is for the removal of carbon dioxide and acid gases from raw natural gas to achieve pipeline quality natural gas (essentially less than 2.5% carbon dioxide). The major component of raw natural gas is methane, with lesser amounts of carbon dioxide, hydrogen sulfide, sulfur oxides, higher hydrocarbons, water, and nitrogen. The nature and purity of the raw gas is dependent on geographic location, geological formation, production history of the well, and the like. The majority of substandard raw gas is purified using chemical sorption systems, but these are costly to build, operate, and maintain. Membrane systems have had limited success in natural gas processing because of high plant investment (a reflection of low membrane permeability), high operating cost (a reflection of low carbon dioxide/methane selectivity), and poor membrane durability (a reflection of polar gas components in the raw gas). Another potential separation is the removal of carbon dioxide from synthesis gas streams, which typically contain hydrogen, carbon dioxide, carbon monoxide, methane and water. Currently, carbon dioxide is removed from synthesis gas by amine absorption, which is a costly and maintenance-intensive process. Existing membranes are permselective to hydrogen, and thus the hydrogen product is obtained at low pressure. It would be highly desirable to maintain the hydrogen at high pressure, which would require a membrane that is permselective to carbon dioxide. The development of a membrane with high carbon dioxide permeability and high carbon dioxide/hydrogen selectivity could significantly reduce the cost of synthesis gas production.
There is relatively little prior art regarding permselective polymeric membranes for separating polar gases from non-polar gases. U.S. Pat. No 5,611,843 discloses a composition suitable for separating gas streams containing carbon dioxide, especially hydrogen rich gas streams containing carbon dioxide and carbon monoxide. The composition comprises a hydrophilic polymer and at least one salt of an amino acid, the salt of the amino acid being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition. The polymers disclosed have to be hydrophilic polymers such as polyvinyl alcohol, polyvinyl acetate, and polyethylene oxide.
Okamoto et al. (Macromolecules, 1995, 28, 6950) reports permeation properties of poly(ether imide) segmented block copolymers for polar/nonpolar gas separations. These polymers consist of hard, glassy polyimide domains and soft, rubbery polyether domains. The polymers have excellent combinations of carbon dioxide permeability and high carbon dioxide/nitrogen separation factors. No data is reported for carbon dioxide/hydrogen separations. Bondar et. al. (Journal of Poly Sci.: Part B, 1999, 37, 2463) shows the gas sorption properties for a family of polyamide-polyether phase separated block copolymers. The gas sorption properties suggest strong favorable interactions between carbon dioxide and the polar linkages in the material, which results in very high carbon dioxide/non-polar gas solubility selectivity in these polymers. Only polyamide-polyether block copolymers are reported, specifically commercially available Pebax®. U.S. Pat. No. 4,963,165 discloses a composite membrane made from a polyamide-polyether block copolymer useful in separating polar gases from non-polar gases. The polymers consist of a saturated aliphatic polyamide hard segment and a polyether soft segment. Only polyamide-polyether block copolymers are reported, specifically commercially available Pebax®.
The membranes of this invention are rubbery in nature and as such exhibit high permeability coefficients, but have low ability to separate gases based on molecular size. However, they exhibit extremely high solubility coefficients for polar gases (e.g., carbon dioxide, hydrogen sulfide, sulfur dioxide, water) and low solubility coefficients for non-polar gases (e.g., helium, hydrogen, nitrogen, methane), and as such offer high solubility selectivity. Thus, the membranes of this invention offer both high permeability and high permselectivity for polar gases. Thus, they are well suited for separation of polar gases from commercial gas mixtures. They are especially suited, without limitation, for removal of polar components from natural gas, and separation of carbon dioxide from synthesis gas.